Finishes that show significant contrast in color and darkness depending upon the viewing angle, often referred to as "polychromism", "flop" or "travel", are currently considered highly desirable for automotive finishes. In general, flop effects in automotive finishes are currently achieved by use of a two coat system using a basecoat and a clear topcoat, wherein the basecoat contains a combination of a transparent organic and/or inorganic pigment with a metal, like aluminum, or a pearlescent, coated mica pigment.
Recently, a new class of opaque, light interference or optically-variable pigments was described, for example, in U.S. Pat. Nos. 4,879,140, 5,059,245, 4,705,300, 5,281,480, 4,434,010, 4,779,898, 4,838,648, 4,390,866, 5,135,812, 5,171,363, 5,214,530, 5,522,923, 5,527,848, 5,607,504 and 4,705,356, which are here incorporated by reference.
Generally, these publications describe pigments prepared by stacking a transparent dielectric layer and semitransparent (partially reflecting) layer on one or both sides of a reflecting or opaque material to form a layered material having the structure EQU M.sub.1 /D/M.sub.2 or M.sub.1 /D/M.sub.2 /D/M.sub.1
where M.sub.2 is an opaque layer, D is a dielectric material and M.sub.1 is a semitransparent layer. Generally, the color and degree of polychromism are controlled mainly by the thickness and identity of the dielectric layer, which influences the interference between the light reflected by the semitransparent layer and that reflected by the opaque layer.
In general, such light interference pigments are described as being produced either by plasma vapor deposition of the layers under high vacuum, or by chemical deposition.
U.S. Pat. No. 5,437,931 discloses that some benefits are derived by protecting the M.sub.1 semitransparent layer with another layer of dielectric material, D', to form a layered material having the structure EQU D'/M.sub.1 /D/M.sub.2 or D'/M.sub.1 /D/M.sub.2 /D/M.sub.1 /D'
This complicated array of layers can be extended even further.
When used in the waterborne basecoats commonly used in the automotive field, metal containing pigments, such as the opaque interference pigments described above, undergo corrosion or oxidation in contact with water, causing undesirable color shifts. U.S. Pat. No. 5,527,848 describes a method to protect the opaque interference pigments from corrosion by outside influences, like water in a water-based paint system, by partial oxidation of the metals contained in the pigment.
EP 668,329, U.S. Pat. No. 5,607,504 and EP 571,836 disclose similar opaque interference pigments which are produced by chemical means whereby aluminum flakes are coated with the dielectric SiO.sub.2 precipitated by hydrolysis of tetraalkoxy silane followed by deposition of a semitransparent metallic layer such as molybdenum, chromium, iron etc. or their oxides by decomposition of the corresponding hexa or penta carbonyl compound and subsequent infusion of oxygen. EP 579,091 describes another variation of this approach whereby the dielectric layer is treated with molybdenum which is subsequently oxidized to its oxide and the latter converted to MoS.sub.2 by treatment with H.sub.2 S. EP 690,105 further discloses that the semitransparent layer can be converted to its nitride by contact with ammonia. These pigments also show strong polychromism and are very opaque and chemically stable.
In this application, interference pigments composed of a light interference layer or light interference layers and an opaque layer, such as those described above wherein a light interference layer consisting of the transparent dielectric material and semitransparent layer is applied to one or both sides of an opaque layer, are referred to as opaque interference pigments in order to distinguish them from metal-oxide coated mica pigments, and the like, which have been used in automotive finishes, in combination with a transparent pigment, for some time. The interference layer acts to generate color and polychromism by causing interference between light reflected from different surfaces, for example the semitransparent and opaque layer surfaces, within the light interference layer.
Opaque interference pigments provide remarkable contrast depending upon the angle of viewing when used in coatings, yet are very opaque, producing saturated colors. This is an oxymoron in terms of conventional pigment technology with oxide-coated mica pigments, which are typically combined with a transparent organic and/or inorganic pigment to achieve the desired pearlescent effects. Compared with conventional finishes containing an oxide-coated mica in combination with a transparent pigment, finishes pigmented with an opaque interference pigment provide unique optical effects, depending upon the viewing angle, not attainable with oxide-coated micas, and excellent outdoor durability. In addition, the opaque light interference pigments are incorporated into a paint vehicle by a simple mixing step, often referred to as stir-in pigments in the automotive industry, which does not require an intense dispersion step; whereas the transparent pigments used in combination with an oxide-coated mica or aluminum flakes generally require intense dispersion steps, which add considerably to cost, for incorporation into a paint vehicle due to its high surface area, and therefore, high degree of aggregation.
The opaque interference pigments are used in paints, inks and plastics. In inks they are used primarily to prevent counterfeiting of currency and other legal documents. The use of opaque interference pigments in two-coat automotive finishes has been described. To impart color and the metallic pearlescent effect to coatings, particularly automotive coatings, significant quantities of the opaque interference pigments have been incorporated in the colored basecoat of a basecoat/clearcoat finish. Such high pigment to binder ratios are necessary to have a basecoat which provides complete hiding. However, the high cost of the opaque interference pigments renders the use of finishes having pigment/binder ratios in the appropriate range too expensive for normal use.
These publications do not disclose that the opaque interference pigment could be incorporated into a midcoat or clearcoat applied over an opaque black, white or colored basecoat at a much lower pigment to binder ratio to achieve remarkable polychromism effects in automotive finishes of widely varying color. In this application, the expressions "colored basecoat" and "colored pigment" are intended to include black and white basecoats or pigments.
It has now been discovered that the amount of the opaque interference pigment required to obtain unusual visual effects is drastically reduced (thus reducing cost), while significantly expanding the available color palette, by depositing onto a substrate a three-coat finish comprising a first coating containing an opaque white, black or colored pigment to achieve total hiding, applying a second coating (midcoat) which contains the opaque interference pigment over the first coat, followed by a conventional clearcoat. The first coating is applied onto the primer, or it can serve as a colored primer. The second coat contains a minor quantity of the opaque interference pigment where the pigment/binder ratio ranges from 0.001/100 up to 7/100 at a thickness from 8 to 40 .mu.m. The second coating does not completely hide the first coating. Thus, the amount of opaque interference pigment applied onto the substrate is dramatically reduced, yet the coated substrate shows unique optical effects resulting from the presence of the opaque interference pigment.
In a further important modification, the substrate is finished with a basecoat/clearcoat finish wherein the clear topcoat having a thickness in the range from 25 to 75 .mu.m is tinted with the opaque interference pigment and applied directly over an opaque black, white or colored basecoat, which in turn is applied onto the primer or which itself serves as a colored primer. In this modification, the midcoat is eliminated, which further reduces cost, while imparting the desired aesthetic effects.
Since the interference pigments utilized according to the present invention are opaque, they are necessarily used in small concentrations to allow the colored basecoat to show through in both the two-coat and three-coat finishing systems described above. Thus, both the two-coat and three-coat finishes vary in color from yellow to orange to red to violet, blue and green, and have a high degree of polychromism.
Compared to the clearcoat/basecoat technology used with oxide-coated mica pigments, the inventive finishes provide for cost-effective, novel effects, such as remarkable polychromism, without the requirement of using a transparent organic and/or inorganic pigment. Eliminating the transparent pigment drastically reduces rheological problems in paint, thereby decreasing requirements for solvents that lead to environmental pollution. Furthermore, transparent pigments require expensive dispersion processes to break up aggregates and agglomerates formed during their manufacture. The invention eliminates the need for this expensive, time-consuming dispersion process by using an opaque pigment in the basecoat which is typically more easily dispersed than a transparent pigment and presents relatively fewer rheological problems. In addition, the invention produces waterborne and solventborne finishes which generally have improved light and weatherfastness compared with finishes containing a transparent pigment.
In addition, opaque interference pigments used according to the present process do not require a treatment, such as that described in U.S. Pat. No. 5,527,848, in order to prevent corrosion of the metal layers when used with waterborne basecoats.